Fuel cells are electrochemical cells in which a free energy change resulting from a fuel oxidation reaction is converted into electrical energy. In an organic/air fuel cell, an organic fuel such as methanol, formaldehyde, or formic acid is oxidized to carbon dioxide at an anode, while air or oxygen is reduced to water at a cathode. Fuel cells employing organic fuels are extremely attractive for portable applications, in part, because of the high specific energy of the organic fuels, e.g., the specific energy of methanol is 6232 Wh/kg.
Direct oxidation fuel cells (as opposed to "indirect" or "reformer" fuel cells) feed the organic fuel directly into the fuel cell and the fuel is oxidized at the anode. Direct oxidation fuel cells use either a vapor or a liquid feed of the organic fuel. Current art direct oxidation fuel cells that show promise typically employ a liquid feed design in which a liquid organic fuel is circulated past the anode of the fuel cell. Liquid feed fuel cells oxidize a fuel such as methanol to produce electricity and chemical by-products including water and protons. In order for the fuel cell to operate properly, the coupled chemical and electrical paths occurring within the device must operate efficiently. Due to the porous nature of the anode, the reaction site for this electro-oxidation reaction is not necessarily juxtaposed to the electrode. As a result, the protons generated by the electro-oxidation reaction are not necessarily transported to the electrolyte and on to the cathode. Any inefficiency in proton transport within the anode will result in reduce electricity generation. One solution to this is to add a mineral acid such as sulfuric acid or phosphoric acid to the methanol fuel. While this alleviates the problem with ionic conduction within the anode, it limits the feasibility of this type of fuel cell by requiring special materials and special handling procedures to deal with the caustic chemical. The use of mineral acids in direct methanol fuel cells presents several problems. Sulfuric acid is highly corrosive and places significant constraints on the materials of construction of the fuel cell, with expensive corrosion resistant materials being required. Sulfate anions, created within the fuel cell, have a strong tendency to adsorb on the electrocatalyst, thereby hindering the kinetics of electro-oxidation of the fuel and resulting in poor performance of the fuel electrode. Also, sulfuric acid tends to degrade at temperatures greater than 80.degree. C. and the products of degradation usually contain sulfur which can poison the electrocatalyst.
An exemplary solid polymer membrane fuel cell using methanol fuel is described in U.S. Pat. No. 5,599,638. U.S. Pat. Nos. 3,013,908 and 3,113,049 describe liquid feed methanol fuel cells using a sulfuric acid electrolyte, and U.S. Pat. Nos. 4,262,063, 4,390,603, 4,478,917 and 4,629,664 describe improvements to sulfuric acid-based methanol fuel cells wherein a high molecular weight electrolyte or a solid proton conducting membrane is interposed between the cathode and the anode as an ionically conducting layer to reduce crossover of the organic fuel from the anode to the cathode. Although the use of the ionically conducting layer helps reduce crossover, the ionically conducting layer is used only in conjunction with a sulfuric acid electrolyte. Hence, these fuel cells suffer from the various aforementioned disadvantages of using sulfuric acid as an electrolyte. In view of the problems associated with using sulfuric acid as an electrolyte, it would be desirable to provide a liquid feed fuel cell that does not require sulfuric acid. Additives such as perfluorooctanesulfonic acid, formaldehyde, trimethoxymethane, dimethoxymethane and trioxane have been proposed to provide improved oxidation rates for organic fuels, but each has a disadvantage.
In general, it is desirable to provide liquid fuels which undergo clean and efficient electrochemical oxidation within the fuel cell. The efficient utilization of organic fuels in direct oxidation fuel cells is, in general, governed by the ease by which the organic compounds are anodically oxidized within the fuel cell. Conventional organic fuels, such as methanol, present considerable difficulties with respect to electro-oxidation. In particular, the electro-oxidation of organic compounds such as methanol involves multiple electron transfer and is a very hindered process with several intermediate steps. These steps involve dissociative adsorption of the fuel molecule to form active surface species which undergo relatively easy oxidation. The ease of dissociative adsorption and surface reaction usually determines the facility of electro-oxidation. Many fuels poison the electrodes of the fuel cell during operation, thus preventing sustained trouble-free operation. Results from ultra high vacuum experiments suggests that the use of higher alcohols such as ethanol will produce CO.sub.2, H+, electrons, and a surface methyl (CH.sub.3) group. This surface methyl group can act as a poison to the electro-oxidation reaction. Likewise, an alcohol containing n carbon atoms (i.e., propanol (C3), butanol (C4), etc.) will release a hydrocarbon fragment to the surface that is n-1 carbon atoms in length. This hydrocarbon fragment can act as a poison for the electro-oxidation reaction by occupying surface reaction sites. As can be appreciated, it would be desirable to provide an improved methanol-type fuel which overcomes the disadvantages of the prior art.